Slave Emperor Sibimet is a boss in BLACKSOULS 2 you can find inside the Queen’s Castle. He can be found on the throne in the room before the cell where Leaf is kept. Sibimet will appear together with his wife, Tabikat. They will both attack you together.
Sibimet is most likely based on a character with the same name from Lewis Carroll’s book Sylvie and Bruno.
If this isn’t your first cycle, it shouldn’t take you a lot of effort to defeat them. Killing him will give you the Soul of the Emperor.
